1. Field of the Invention
The present invention relates to an illumination system and a projector adopting the same, and more particularly, to an illumination system which realizes a color image without a color wheel by using a light emitting device or light emitting device array, and a projector adopting the same.
2. Description of the Related Art
Referring to FIG. 1, a conventional single panel projector includes a light source 100, a first relay lens 102 which condenses a light beam emitted from the light source 100, a color wheel 105 which splits an incident light beam into three light beams of red (R), green (G) and blue (B) colors, a fly eye lens 107 which makes the light beam passing through the color wheel 105 uniform, a second relay lens 110 which condenses the light beam passing through the fly eye lens 107, a display device 112 which forms a color image from the R, G and B color beams which are sequentially incident after passing through the color wheel 105, and a projection lens unit 115 which magnifies the image formed by the display device 112 and projects the magnified image onto a screen 118.
A xenon lamp, a metal-halide lamp, or a UHP lamp is used as the light source 100. These lamps emit excess infrared and ultraviolet rays. Accordingly, a great amount of heat is generated, and a cooling fan is necessary. However, the cooling fan is a source of noise. Also, since the spectrum of the light source lamp is widely distributed across the entire wavelength spectrum, due to a narrow color gamut, the selection of color is limited, color purity is inferior, and a life span of the lamp is short, making stable use of the lamp undesirable.
In the conventional single panel projector, to realize a color image, the color wheel 105 is rotated by a driving motor (not shown) at a high speed so that R, G and B color light beams are sequentially incident onto the display device 112. However, since R, G and B color filters are equally arranged over the entire surface of the color wheel 105, and the filters are used one by one during the rotation of the color wheel 105, according to a response speed of the display device 112, ⅔ of the light emitted by the light source 100 is lost. Also, since a gap of a predetermined width separates neighboring color filters for preferable generation of color, light is also lost at the gap.
Since the color wheel 105 rotates at a high speed, noise is generated. Also, the mechanical movement of the driving motor has a bad effect on stability, and, due to mechanical limits in the driving motor, it is difficult to obtain speeds past a certain point, and a color breakup phenomenon occurs. Further, since the color wheel is very expensive, the manufacturing cost of a projector rises.
To solve the above-described problems, it is an object of the present invention to provide an illumination system having a light emitting device or light emitting device array and a holographic optical element, to improve color purity and color selection and to realize a color image without a color wheel, and a projector adopting the illumination system.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
To achieve the above and other objects of the invention, there is provided an illumination system comprising at least one light emitting device which emits a light beam having a predetermined wavelength, and a holographic optical element which reduces the cross section of the light beam emitted from the light emitting device.
The light emitting device may comprise one of an LED (light emitting diode), an LD (laser diode), an organic EL (electro luminescent), and an FED (field emission display).
The illumination system may further comprise a parallel light beam forming unit which makes the light beam emitted from the light emitting device a parallel light beam.
The parallel light beam forming unit may further comprise a collimating lens array or a Fresnel lens array.
The illumination system may further comprise an optical path changing unit which changes a path of a light beam by selectively transmitting or reflecting the light beam exiting from the holographic optical element.
A plurality of light emitting devices may be arranged in a form of an array.
A plurality of light emitting devices or light emitting device arrays which emit light beams having different wavelengths may be arranged in a line.
The optical path changing unit may be a dichroic filter which reflects or transmits the light beam exiting from the holographic optical element according to the wavelength of the light beam.
The optical path changing unit may be a cholesteric band modulation filter which reflects or transmits the light beam exiting from the holographic optical element according to a polarization direction and a wavelength of the light beam.
The cholesteric band modulation filter may comprise a first mirror surface which reflects a light beam of right circular polarization and transmits a light beam of left circular polarization, and a second mirror surface which transmits a light beam of right circular polarization and reflects a light beam of left circular polarization, with respect to a light beam having a predetermined wavelength.
A plurality of light emitting devices or light emitting device arrays which emit light beams having different wavelengths may be arranged to be separated by a predetermined angle.
The optical path changing unit may be an X prism or X type dichroic filter.
The light emitting device or the light emitting device array and the holographic optical element may be further arranged in a multiple layer structure.
To achieve the above and other objects of the present invention, there is provided a projector comprising an illumination system; a display device which forms an image by processing a light beam emitted from the illumination system according to an input image signal; and a projection lens unit which magnifies the image formed by the display device and projects the magnified image onto a screen; wherein the illumination system comprises at least one light emitting device which emits a light beam having a predetermined wavelength, and a holographic optical element arranged on an optical path of the emitted light beam which reduces a cross section of the light beam emitted from the light emitting device.
The projector may further comprise a fly eye lens which makes the light beam emitted from the illumination system a parallel beam, and a relay lens which condenses the light beam passing through the fly eye lens toward the display device.